Compressed air systems are used to provide energy for driving a variety of devices in a variety of applications. One such application is a railroad locomotive where compressed air is used to power locomotive air brakes and pneumatic control systems.
A typical compressed air system will include a reservoir for storing a volume of compressed air. A motor-driven compressor is used to maintain the air pressure in the reservoir within a desired range of pressures. The reservoir pressure may be higher than the demand pressure for a device supplied by the system, in which case a pressure regulator may be used to reduce the pressure supplied to the device. The stored volume of compressed air in the reservoir provides an inertia that allows the compressor to be sized smaller than would otherwise be necessary if the compressor supplied the individual devices directly. Furthermore, the stored volume of compressed air in the reservoir allows the compressor to be cycled on and off less frequently than would otherwise be necessary in a direct-supply system. This is important because the electrical and mechanical transients that are generated during a motor/compressor start-up event may severely challenge the compressor motor and associated electrical contacts.
The size and operating pressures of the compressor and reservoir in a compressed air system are matters of design choice. A larger, higher-pressure reservoir will reduce the duty cycle of the compressor motor, but there are associated cost, size and weight constraints that must be considered. Furthermore, the control system set points used to control the compressor starts and stops may be varied within overall system limits. Compressed air systems for locomotives are designed with the benefit of experience accumulated during the operation of generations of locomotives. However, in spite of the optimization of system design, there have been instances of specific operating conditions unique to a particular locomotive or group of locomotives that result in an undesirably high duty cycle for the air compressor motor. Because such locomotive-specific conditions may be transient and may not be representative of conditions experienced by an entire fleet of locomotives, it is not necessarily desirable to further refine the compressed air system components in response to such conditions. Thus, a compressed air system that is less susceptible to excessive cycling of the compressor motor is desired.